The invention relates to a circuit arrangement for generating an x-ray tube voltage, having an inverse rectifier circuit for generating a high-frequency alternating voltage, having a high-voltage generator for converting the high-frequency alternating voltage into a high voltage for the x-ray tube, and having a voltage controller, which on the basis of a deviation of an actual x-ray tube voltage from a set-point x-ray tube voltage generates a first controlling variable value for a controlling variable for the inverse rectifier circuit, for achieving an adaptation of the actual x-ray tube voltage to the set-point x-ray tube voltage. One such circuit arrangement is known from German Patent DE 29 43 816 C2.
The invention also relates to an x-ray generator having such a circuit arrangement, an x-ray system having such an x-ray generator, and a corresponding method for generating an x-ray tube voltage.
To generate an x-ray tube voltage, modern x-ray generators often have circuit arrangements of the typed defined at the outset. Since a line frequency is first rectified and then converted back into a high-frequency alternating voltage that is finally transformed to a desired voltage, such generators are also known as high-frequency generators. The voltage controller serves to regulate the high voltage at the x-ray tube as optimally as possible in terms of time to a diagnostically required value with a requisite precision.
Compared to conventional generators, in which the high voltage is first transformed using the line frequency present, then rectified, and finally delivered to the x-ray tube, such a circuit arrangement has the advantage that in principle, it can be made virtually independent of changes to a line voltage and to a tube current by means of a relatively fast closed-loop current circuit. The tube voltage is therefore highly replicable and can be kept constant. Compared to so-called direct voltage generators, in which a high voltage, transformed at line frequency and rectified, is finely regulated with the aid of triodes, high-frequency generators have the advantage of a relatively small structural volume and lower production costs. These advantages are the reason for the preferred use of such circuit arrangements in modern x-ray generators.
In conventional circuit arrangements of the type defined at the outset problems arise from the fact that parameters of a controlled system including an inverse rectifier and the high-voltage circuit, depending on the selected operating point of the x-ray tube, cover a wide range of values, and that in particular the inverse rectifier""s resonance characteristic is a highly nonlinear member of the closed-loop control circuit. Moreover, if damage to the power semiconductor is to be avoided, an oscillating current of the inverse rectifier must not exceed a predetermined maximum value. In a conventional single-step x-ray tube voltage closed-loop control circuit, a control speed of the circuit must therefore be set to be at least slow enough that the oscillating circuit current, even during running up to speed or when being turned on, does not exceed the maximum allowable value. As a result, a small-signal behavior of the closed-loop control circuit is also slowed down, resulting in a slower elimination of interference variables than would intrinsically be possible. Moreover, with this kind of a single-step control, the oscillating current is limited only indirectly. Therefore if the inverse rectifier is redimensioned, the control parameters of the controller must be adapted to suit the oscillating current. A simple voltage controller can thus meet the demands, even if only to an unsatisfactory extent.
It is therefore the object to create an alternative to the known prior art that permits high-speed control without exceeding the maximum allowable oscillating current.
This object is attained by a circuit arrangement as defined by claim 1 and by a method as defined by claim 9.
To that end, the circuit arrangement additionally has a measurement circuit for measuring an oscillating current, applied to one output of the inverse rectifier circuit, of the high-frequency alternating voltage. By means of an oscillating current controller, a second controlling variable value for the aforementioned controlling variable of the inverse rectifier circuit is then generated on the basis of a deviation of an ascertained actual oscillating current value from a predetermined maximum oscillating current value. The voltage controller and the oscillating current controller are then coupled in series to a switching device, which compares a first controlling variable value and a second controlling variable value and forwards only the lesser of the two controlling variable values, as the resultant controlling variable value, to the inverse rectifier circuit.
A second controlling variable value is ascertained separately by means of an oscillating current controller on the basis of the deviations of an actual oscillating current value from a predetermined maximum oscillating current value and compared with the first controlling variable value of the voltage controller, and only the lesser of the two controlling variable values is delivered to the inverse rectifier circuit. It is attained that in a normal situation, very fast control by the voltage controller is accomplished; and only in extreme cases, if a critical range for the oscillating current is attained, is the voltage controller relieved by the oscillating current controller. In other words, in this xe2x80x9crelief controlxe2x80x9d, as long as the voltage controller is functioning xe2x80x9cnormallyxe2x80x9d and provides only an oscillating current that is less than the maximum allowable oscillating current, the controlling variable of the voltage controller will be sent on to the controlled system. Only if the maximum allowable oscillating current is reached or exceeded, which will be the case for instance during running up to speed as a rule, does the oscillating current controller come into play and limit the oscillating current to its maximum allowable value.
The dependent claims contain various especially advantageous features and refinements of the invention.
Preferably, for at least one of the two controllers and especially preferably for both controllers, a PI controller (proportional-integral controller) is used. An integral portion of the applicable controller has an object of forcing a steady-state control error, that is a control error in a steady state, to zero. Thus a persistent control deviation is reliably avoided. The controllers preferably then comprise series-connected proportional parts and integral parts. The advantage over a parallel PI controller structure is that now the controller parameters pertaining to an amplification and an adjustment or a readjustment time can be set separately from one another. Instead of a PI controller, a PID controller can also be used.
In an especially preferred exemplary embodiment, an output of the switching device is connected to one input of the voltage controller and/or of the oscillating current controller, for feeding back the resultant controlling variable value. The voltage controller and/or the oscillating current controller are embodied such that they forward the resultant controlling variable value, if the controlling variable value generated by the applicable controller is not forwarded as the resultant controlling variable value.
To that end, the applicable controller compares the resultant controlling variable with its own controlling variable value that is internally also fed back. As a result of this variant, additional transient events caused by abrupt changes or surges upon switchover between the two controllers are reliably prevented.
Preferably, the switching device is embodied such that it sends at least a predetermined minimum controlling variable value as the resultant controlling variable value onward to the inverse rectifier circuit. Moreover, preferably at most, a predetermined maximum controlling variable value is sent onward, as the resultant controlling variable value, to the inverse rectifier circuit. Hence, the result controlling variable is actively limited to a range between the minimum value and the maximum value.
Since the controller parameters, being the controller amplification and the readjustment time, are as a rule dependent on the operating point, the voltage controller and/or the oscillating current controller preferably can each vary at least one parameter (i.e., controller parameter) of the applicable controller as a function of a set x-ray tube voltage and/or as a function of a set x-ray tube current. That parameter is then fed to corresponding inputs of the respective controller, and as a result the parameters of the applicable controllers are suitably set internally.
A circuit arrangement according to the invention can in principle be used to generate an x-ray tube voltage in any conventional x-ray generator, regardless of how the x-ray generator is constructed in terms of its further components, such as the various measuring instruments or the supply of heating current. The invention can also be employed largely independently of the concrete embodiment of the inverse rectifier circuit and of the high-voltage generator.